(a) Field of the Invention
The present invention relates to an optical waveguide composed of a high-silica material.
(b) Description of the Related Art
In these days, advanced multi-functional optical systems are demanded with the increase of the capacity of the optical communication system. There are also a demand for lowering costs of optical fiber networks. Optical waveguides are a requisite for miniaturization, higher integration and low cost production of optical devices, hence various studies have been carried out on hybrid functional devices in which a semiconductor laser, a semiconductor photo detector etc. are mounted on a substrate together with optical waveguides, as well as on devices composed of optical waveguides only, such as an array of couplers, switches, filters and modulators. A specific structure of a hybrid optical device has been proposed, for example, in a literature presented by Henry et al., "Lightwave Technology", IEEE, pp. 1530-1539, 1989.
An optical waveguide composed of a high-silica material (or silica-based material) and formed on an Si substrate generally comprises three layers including a lower cladding layer, a core, and an upper cladding layer. In order to obtain an optical waveguide having a propagation loss sufficiently small in a practical optical communication system, each cladding layer should have a thickness more than about 10 .mu.m. The core also should have a thickness more than about 5 .mu.m in an optical device to obtain an efficient coupling with an optical fiber in the wave band for the optical communication system. Accordingly, at least about 25 .mu.m is required for the total thickness of the three layers for obtaining an optical waveguide having a propagation loss sufficiently small in the practical optical communication system.
FIG. 1 shows a cross section of a conventional optical waveguide formed on an Si substrate 1 by a flame hydrolysis deposition using a chloride gas or CVD method using a silane gas or chloride gas for depositing a high-silica material. In flame hydrolysis deposition, undoped silica is used for a cladding 7, and a high-silica material is used for a core 8 in which a single dopant of germanium oxide (GeO.sub.2) or titanium oxide (TiO.sub.2) is introduced, or in which both phosphorus oxide (P.sub.2 O.sub.5) and boron oxide (B.sub.2 O.sub.3) are introduced. In case of flame hydrolysis deposition, silica is made transparent by a thermal treatment at a temperature about 1500.degree. C. after silica powder has been deposited on the substrate 1. In this thermal treatment, however, cracks are likely to be generated in the high-silica layer due to the thermal strains, which makes it difficult to apply the flame hydrolysis deposition process to a large Si substrate of, for example, 6 inches in diameter. Furthermore, in an optical networks including a semiconductor laser etc. mounted on the common Si substrate together with an optical waveguide, it should be avoided to form metal layers such as electrodes or interconnections for the semiconductor laser on the Si substrate before the thermal treatment of the cladding, in view of the melting point of the metal pattern.
On the other hand, as to the CVD method, a silica material doped with a single dopant P.sub.2 O.sub.5 or GeO.sub.2 is widely used in the CVD process. In this process, a thermal treatment at about 1000.degree. C. is conducted for allowing the high-silica material to reflow at the boundary between the core 8 and the cladding 7 in order to improve the contact between the core 8 and the cladding 7 and to densify the layers thereby obtaining an optical waveguide having a small propagation loss. As a result, the thermal strain is as large in the CVD method as in the flame hydrolysis deposition, so that cracks are likely to be generated in the silica layer, which makes it difficult to apply the CVD method to a large Si substrate of, for example, 6 inches in diameter.
It is known that the reflow point of a silica material goes down to a temperature ranging from 850.degree. to 900.degree. C. when doped with P.sub.2 O.sub.5 or B.sub.2 O.sub.3 during a CVD process. Since the thermal strain would be drastically reduced if the temperature range of the thermal treatment could be further reduced, an Si substrate having a larger diameter could be used in the production of waveguides to improve the propagation loss thereof. Furthermore, with a hybrid device including a semiconductor laser mounted on the Si substrate, such a temperature range would enable the metal layers to be formed on the Si substrate before the thermal treatment of the cladding so that mass production of hybrid optical devices and reduction of production cost could be realized.